Bearing rings used in bearings are locally and repetitively subjected to high surface pressure, and are required to have wear resistance. A high-carbon chromium bearing steel specified by JIS G 4805 is used in bearing rings of such bearings. This steel is made by increasing a carbon content of base metal, and applying heat treatment for spheroidizing (spheroidizing annealing) carbide, so as to attain a hard surface, preferable sliding characteristics and machinability.
In such a high-carbon chromium bearing steel, the base metal has a high C content, and its surface becomes decarburized when heated at a high temperature unless a furnace atmosphere is controlled. Such a steel for use in bearing steel is usually heated at a high temperature in a decarburizing atmosphere during processes of casting, blooming and billet-making, and hot tube-making; and consequently, a decarburized layer still remains on the surface of steel after a rolling process (cold rolling, cold drawing).
A high-carbon chromium bearing steel is subjected to processing such as cutting-off, machining, and quenching, and used for producing parts such as bearing rings, and if a decarburized layer remains after machining, a predetermined strength and microstructure cannot be attained in a portion with the decarburized layer, which causes deterioration of sliding characteristics. Consequently, if there is a deep decarburized layer, this decarburized layer is removed by use of grinding or the like before machining, which causes increase in man-hour, and deterioration of a yield, resulting in significant increase in a manufacturing cost.
As a solution for the problem, disclosed are heat treatment methods for recarburizing the decarburized layer through an atmosphere control. For example, Patent Literature 1 describes carburizing in an austenite region at a high temperature. A spheroidizing annealing method is disclosed in which bearing steel or hypereutectoid steel is used as a target, wherein, the target material is heated to be carburized in an austenite (γ) region of not less than an Acm transformation point at an initial stage of a heat pattern of spheroidizing annealing, thereby preventing decarburization, which has been generated in conventional spheroidizing annealing.
Meanwhile, Patent Literature 2 and Patent Literature 3 describe carburizing in a ferrite region. Patent Literature 2 proves that diffusion velocity of carbon is further rapid in α phase at a lower temperature than in γ phase at a higher temperature, and recarburizing can be sufficiently attained even in α phase, and also Patent Literature 2 discloses a carburizing and heat treatment method of high-carbon chromium bearing steel, wherein during spheroidizing annealing, recarburizing and spheroidizing of a decarburized layer are carried out at the same time in a carburizing atmosphere in a temperature range of 720 to 700° C. that is not more than A1 transformation point.
Patent Literature 3 discloses such a heat treatment method that controls a temperature in discharging a workpiece, an atmosphere gas flow rate at a discharging zone and an furnace pressure, a cooling rate, and a (CO)2/CO2 value of an atmosphere in a temperature range of not more than A1 transformation point to not less than 500° C., thereby preventing generation of soot, and reducing a decarburized layer owing to recarburization.
Patent Literature 4 describes a decarburization level control through gas carburizing and carbon diffusion. Specifically, Patent Literature 4 discloses such a decarburization suppressing method of high-carbon chromium bearing steel, wherein recarburizing and overcarburizing are carried out at the same time as spheroidizing in an atmosphere-controlled annealing furnace, and continuously spheroidizing is carried out in a direct fired heating furnace while sufficiently encouraging scale-off and carbon diffusion in an overcarburized portion of steel, thereby controlling the decarburization level.
Patent Literature 5 describes recarburizing after descaling. Specifically, Patent Literature 5 discloses such a recarburizing method of a steel ring cut from a high-carbon chromium bearing steel tube, wherein oxygen is removed through descaling prior to heat treatment in a carburizing atmosphere so as to facilitate recarburizing, and thereafter, annealing is applied in the carburizing atmosphere for recarburizing or overcarburizing the steel ring.
Patent Literature 6 and Patent Literature 7 describe decarburizing prevention by removing oxygen in scale or removing the scale. Patent Literature 6 discloses such a heat treatment method of steel, wherein (CO)2/CO2 of the atmosphere is controlled so as to remove oxygen in the scale prior to final heating, and an atmosphere control is carried out without recarburizing or decarburizing in the final heat treatment. Patent Literature 7 discloses such a decarburizing prevention method of an inner surface of a steel tube, wherein spheroidizing annealing of the steel tube is carried out using a direct fired heat treatment furnace by applying annealing to the steel tube with its both tube ends sealed after the scale is removed. It is considered that an inner surface of the workpiece has no reaction with a furnace atmosphere gas because carbon diffusion is encouraged inside the workpiece.
Patent Literature 8 describes decarburizing prevention of an inner surface of a steel tube by coating graphite to this inner surface. Specifically, Patent Literature 8 discloses a decarburizing preventing method of the inner surface of the steel tube, in which in heat treatment of the steel tube in a decarburizing atmosphere furnace, a mixture of graphite and non-flammable liquid even at a high temperature is applied on the inner surface of the steel tube, so as to increase carbon potential on the inner surface of the steel tube, thereby carrying out recarburizing of the steel tube.